The environment to which humans are exposed often contains toxins and stress factors, which induce a variety of biological responses. These are gene-directed active processes; thus, how the cells respond and how genes and environments interact play an important role in disease development. The goal of this R21 is to establish a mouse model to study the biological responses to various environmental stresses. Designs and Utilities: The design is such that it will allow the researchers to trace and study both the cells under stress at the time of harvesting the tissues and the cells that were under stress in their past - long before harvesting. The mice will carry two transgenic alleles. 1. Stress-inducible Cre*: encodes a recombinase Cre fusion (Cre* for short), whose expression is turned on by a broad spectrum of stress signals, and 2. ROSA-Reporter: encodes Yellow Fluorescent Protein (YFP) preceded by Lox-STOP-Lox (LSL). The production of YFP is blocked by LSL and is possible only if the STOP is removed by active Cre*. This event physically and thus permanently alters the DNA, resulting in YFP production (green) to forever mark the cells. This allows one to trace the cells and determine their fate. The Cre* is fluorescent (red); thus, the cells can be labeled red, if they are actively under stres (producing Cre*). With proper designs, one can distinguish cells that were once under stress in their past from those currently under stress. The fluorescent colors allow not only cell imaging but also cell isolation (by sorting) for biochemical and molecular analyses. Aim 1: To generate and characterize the stress-inducible Cre* transgenic mice. The transgenic construct will be generated using BAC recombineering and injected into pronucleus to make the mice. The mice will then be characterized for the expression and half-life of Cre* under acute and chronic stress paradigms. Aim 2: To generate and characterize the double transgenic mice. The stress-inducible Cre* mice will be crossed with the YFP reporter mice (available) to generate the double transgenic mice. The mice will then be analyzed to determine how tight the system is and how efficient the active Cre* is in removing the STOP. Innovation and Significance: The mouse model is novel for its ability to trace the fate of cells under various stresses, such as those induce liver injury, heart attack, neuronal degeneration, and cancer development. In addition, the proposed mouse model allows researchers to address issues regarding the transition from acute to chronic (or repeated) injuriesan important but not well understood issue. The broad inducibility of Cre* makes the model an excellent tool to study environmentally-induced diseases - a special emphasis of NIEHS.